This invention generally relates to centrifugal vapor compressors, and more particularly to diffuser valve assemblies therefor.
One of the major difficulties arising in the use of centrifugal vapor compressors for applications where the compressor load varies over a wide range is flow stabilization through the compressor. The compressor inlet, impeller, and diffuser passages must be sized to provide for the maximum desired volumetric flow rate. When there is a low volumetric flow rate through such a compressor, the flow may become unstable. That is, as the volumetric flow rate is decreased from a stable range, a range of slightly unstable flow is entered. In this range, there appears to be a partial reversal of flow in the diffuser passage, creating noises and lowering compressor efficiency. Below this range, the compressor enters what is known as surge, wherein there are periodic complete flow reversals in the diffuser passage, destroying the efficiency of the machine and endangering the integrity of the machine elements. Since a wide range of volumetric flow rates is desirable in many compressor applications, numerous modifications have been employed to improve flow stability at low volumetric flow rates. One widely known modification is to vary diffuser passage configuration in response to changes in the load on the compressor. Frequently, this is done by means of an annular diffuser valve which laterally moves across the diffuser passage to throttle vapor flow therethrough. More specifically, the compressor housing is provided with an annular recess in communication with the diffuser passage, and the diffuser valve is supported by the housing for movement within the annular recess and the diffuser passage. Means are provided to move the diffuser valve across the diffuser passage, thus throttling vapor flow therethrough. Often, a stop is provided to limit lateral movement of the diffuser valve at a maximum throttling position and maintain a minimum flow space through the diffuser passage.
The preferred maximum throttling position of the diffuser valve may vary from one machine to another, depending upon the specific application of the machine. Similarly, if the particular application of a compressor is changed, then the preferred maximum throttling position of a diffuser valve thereof may also change. With many types of compressors, providing otherwise identical compressors with diffuser valves having different maximum throttling positions, or varying the maximum throttling position of a particular diffuser valve has heretofore involved notable difficulties. For example, with many compessors, employing prior art valve stops which can be adjusted to change the maximum throttling position of the diffuser valve may interfere with the design or operation of other parts of the diffuser asembly such as the diffuser valve itself or the means used to move the valve across the diffuser passage. As a result, such compressors generally do not employ adjustable stops, and changing or adjusting the maximum throttling position of these compressors may require replacing a major, expensive part thereof, for example an entire wall or diaphragm of the compressor.